High modulus crosslinked polyethylene with reduced residual free radical concentration prepared below the melt

ABSTRACT

The present invention provides an irradiated crosslinked polyethylene containing reduced free radicals, preferably containing substantially no residual free radical. Disclosed is a process of making irradiated crosslinked polyethylene by irradiating the polyethylene in contact with a sensitizing environment at an elevated temperature that is below the melting point, in order to reduce the concentration of residual free radicals to an undetectable level. A process of making irradiated crosslinked polyethylene composition having reduced free radical content, preferably containing substantially no residual free radicals, by mechanically deforming the polyethylene at a temperature that is below the melting point of the polyethylene, optionally in a sensitizing environment, is also disclosed herein.

[0001] This application claims priority to provisional applicationSerial No. 60/344,354, filed Jan. 4, 2002, the entirety of which ishereby incorporated by reference.

[0002] The present invention relates to irradiated crosslinkedpolyethylene (PE) compositions having reduced free radical content,preferably containing reduced or substantially no residual freeradicals, and processes of making crosslinked polyethylene. Theprocesses can comprise the steps of irradiating the polyethylene whileit is in contact with a sensitizing environment at an elevatedtemperature that is below the melting point in order to reduce theconcentration of residual free radicals, preferably to an undetectablelevel. The invention also relates to processes of making crosslinkedpolyethylene having reduced free radical content, preferably containingsubstantially no residual free radicals, by mechanically deforming theirradiated PE either with or without contact with sensitizingenvironment during irradiation, at a temperature that is below themelting point of the polyethylene. These processes are complementary andcan be used together or separately.

DESCRIPTION OF THE FIELD

[0003] Increased crosslink density in polyethylene is desired in bearingsurface applications for joint arthroplasty because it significantlyincreases the wear resistance of this material. The preferred method ofcrosslinking is by exposing the polyethylene to ionizing radiation.However, ionizing radiation, in addition to crosslinking, also willgenerate residual free radicals, which are the precursors ofoxidation-induced embrittlement. This is known to adversely affect invivo device performance. Therefore, it is desirable to reduce theconcentration of residual free radicals, preferably to undetectablelevels, following irradiation to avoid long-term oxidation.

[0004] In the past, in order to substantially reduce the concentrationof residual free radicals in irradiated polyethylene, the polyethylenehas to be heated to above its melting temperature (for example, about140° C.). Melting frees or eliminates the crystalline structure, wherethe residual free radicals are believed to be trapped. This increase inthe free radical mobility facilitates the recombination reactions,through which the residual free radical concentration can be markedlyreduced. This technique, while effective at recombining the residualfree radicals, has been shown to decrease the final crystallinity of thematerial. This loss of crystallinity will reduce the modulus of thepolyethylene. Yet for high stress applications, such as unicompartmentalknee designs, thin polyethylene tibial knee inserts, low conformityarticulations, etc., high modulus is desired to minimize creep.

[0005] It is therefore desirable to reduce the residual free radicalconcentration without heating above the melting point in order to avoidsignificantly reducing the crystallinity of polyethylene, so as topermit insubstantial lowering, substantial maintenance, or an increasein the modulus.

SUMMARY OF THE INVENTION

[0006] An object of the invention to provide an improved irradiatedcrosslinked polyethylene having reduced concentration of free radicals,made by the process comprising irradiating the polyethylene at atemperature that is below the melting point of the polyethylene,optionally while it is in contact with a sensitizing environment, inorder to reduce the content of free radicals, preferably to anundetectable level, optionally through mechanical deformation.

[0007] In accordance with one aspect of the present invention, there isprovided an irradiated crosslinked polyethylene wherein crystallinity ofthe polyethylene is at least about 51% or more.

[0008] In accordance with another aspect of the present invention, thereis provided an irradiated crosslinked polyethylene, wherein the elasticmodulus of the polyethylene is higher or just slightly lower than, i.e.about equal to, that of the starting unirradiated polyethylene orirradiated polyethylene that has been subjected to melting.

[0009] According to the present invention, the polyethylene is apolyolefin and preferably is selected from a group consisting of alow-density polyethylene, high-density polyethylene, linear low-densitypolyethylene, ultra-high molecular weight polyethylene (UHMWPE), ormixtures thereof.

[0010] In one aspect of the present invention, the polyethylene iscontacted with a sensitizing environment prior to irradiation. Thesensitizing environment, for example, can be selected from the groupconsisting of acetylene, chloro-trifluoro ethylene (CTFE),trichlorofluoroethylene, ethylene or the like, or a mixture thereofcontaining noble gases, preferably selected from a group consisting ofnitrogen, argon, helium, neon, and any inert gas known in the art. Thegas can be a mixture of acetylene and nitrogen, wherein the mixturecomprising about 5% by volume acetylene and about 95% by volumenitrogen, for example.

[0011] In one aspect of the invention, the starting material of thepolyethylene can be in the form of a consolidated stock or the startingmaterial can be also in the form of a finished product.

[0012] In another aspect of the invention, there is provided anirradiated crosslinked polyethylene with reduced free radicalconcentration, preferably with no detectable residual free radicals(that is, the content of free radicals is below the current detectionlimit of 10¹⁴ spins/gram), as characterized by an elastic modulus ofabout equal to or slightly higher than that of the starting unirradiatedpolyethylene or irradiated polyethylene that has been subject tomelting. Yet in another aspect of the invention, there is provided acrosslinked polyethylene with reduced residual free radical content thatis characterized by an improved creep resistance when compared to thatof the starting unirradiated polyethylene or irradiated polyethylenethat has been subjected to melting.

[0013] In accordance with one aspect of the invention there is provideda method of making a crosslinked polyethylene comprising irradiating thepolyethylene at a temperature that is below the melting point of thepolyethylene while it is in contact with a sensitizing environment inorder to reduce the content of free radicals, preferably to anundetectable level.

[0014] In accordance with another aspect of the invention, there areprovided methods of treating crosslinked polyethylene, whereincrystallinity of the polyethylene is about equal to that of the startingunirradiated polyethylene, wherein crystallinity of the polyethylene isat least about 51% or more, wherein elastic modulus of the polyethyleneis about equal to or higher than that of the starting unirradiatedpolyethylene or irradiated polyethylene that has been subjected tomelting.

[0015] There also is provided a method of making a crosslinkedpolyethylene, wherein the annealing temperature is below the meltingpoint of the polyethylene, wherein the annealing temperature is lessthan about 145° C., preferably less than about 140° C. and morepreferably less than about 137° C.

[0016] Also provided herein, the material resulting from the presentinvention is a polyethylene subjected to ionizing radiation with reducedfree radical concentration, preferably containing substantially noresidual free radicals, achieved through post-irradiation annealing atbelow the melting point at less than 145° C., preferably at less than140° C. and more preferably at less than 137° C., in the presence of asensitizing environment.

[0017] In one aspect of the invention, there is provided a method ofmaking a crosslinked polyethylene, wherein the polyethylene is contactedwith a sensitizing environment prior to irradiation.

[0018] In another aspect according to the present invention, there isprovided a method of making a crosslinked polyethylene, wherein thesensitizing environment is acetylene, chloro-trifluoro ethylene (CTFE),trichlorofluoroethylene, ethylene gas, or mixtures of gases thereof,wherein the gas is a mixture of acetylene and nitrogen, wherein themixture comprises about 5% by volume acetylene and about 95% by volumenitrogen.

[0019] Yet in another aspect according to the present invention, thereis provided a method of making a crosslinked polyethylene, wherein thesensitizing environment is dienes with different number of carbons, ormixtures of liquids and/or gases thereof.

[0020] One aspect of the present invention is to provide a method ofmaking a crosslinked polyethylene, wherein the irradiation is carriedout using gamma radiation or electron beam radiation, wherein theirradiation is carried out at an elevated temperature that is below themelting temperature, wherein radiation dose level is between about 1 andabout 10,000 kGy.

[0021] In one aspect there is provided a method of making a crosslinkedpolyethylene, wherein the annealing in the presence of sensitizingenvironment is carried out at above an ambient atmospheric pressure ofat least about 1.0 atmosphere (atm) to increase the diffusion rate ofthe sensitizing molecules into polyethylene.

[0022] In another aspect there is provided a method, wherein theannealing in the presence of sensitizing environment is carried withhigh frequency sonication to increase the diffusion rate of thesensitizing molecules into polyethylene.

[0023] Yet in another aspect there is provided a method of treatingirradiated crosslinked polyethylene comprising steps of contacting thepolyethylene with a sensitizing environment; annealing at a temperaturethat is below the melting point of the polyethylene; and elevating thetemperature that is below the melting point in presence of a sensitizingenvironment in order to reduce the concentration of residual freeradicals, preferably to an undetectable level.

[0024] Another aspect of the invention provides an improved irradiatedcrosslinked polyethylene composition having reduced free radicalconcentration, made by the process comprising irradiating at atemperature that is below the melting point of the polyethylene,optionally in a sensitizing environment; mechanically deforming thepolyethylene in order to reduce the concentration of residual freeradical and optionally annealing below the melting point of thepolyethylene, preferably at about 135° C., in order to reduce thethermal stresses.

[0025] In accordance with one aspect of the invention, mechanicaldeformation of the polyethylene is performed in presence of asensitizing environment at an elevated temperature that is below themelting point of the polyethylene, wherein the polyethylene has reducedfree radical content and preferably has no residual free radicalsdetectable by electron spin resonance.

[0026] In accordance with another aspect of the invention theirradiation is carried out in air or inert environment selected from agroup consisting of nitrogen, argon, helium, neon, and any inert gasknown in the art.

[0027] In accordance with still another aspect of the invention, themechanical deformation is uniaxial, channel flow, uniaxial compression,biaxial compression, oscillatory compression, tension, uniaxial tension,biaxial tension, ultra-sonic oscillation, bending, plane stresscompression (channel die) or a combination of any of the above andperformed at a temperature that is below the melting point of thepolyethylene in presence or absence of a sensitizing gas.

[0028] Yet in accordance with another aspect of the invention,mechanical deformation of the polyethylene is conducted at a temperaturethat is less than the melting point of the polyethylene and above roomtemperature, preferably between about 100° C. and about 137° C., morepreferably between about 120° C. and about 137° C., yet more preferablybetween about 130° C. and about 137° C., and most preferably at about135° C.

[0029] In one aspect, the annealing temperature of the irradiatedcrosslinked polyethylene is below the melting point of the polyethylene,preferably less than about 145° C., more preferably less than about 140°C., and yet more preferably less than about 137° C.

[0030] Yet in another aspect, there is provided an irradiatedcrosslinked polyethylene, wherein elastic modulus of the polyethylene isabout equal to or higher than that of the starting unirradiatedpolyethylene.

[0031] In accordance with the present invention, there is provided amethod of making an irradiated crosslinked polyethylene comprisingirradiating at a temperature that is below the melting point of thepolyethylene, optionally in a sensitizing environment; mechanicallydeforming the polyethylene in order to reduce the concentration ofresidual free radical and optionally annealing below the melting pointof the polyethylene, preferably at about 135° C., in order to reduce thethermal stresses.

[0032] In accordance with one aspect of the invention, there is provideda method of mechanical deformation of polyethylene, optionally inpresence of a sensitizing environment, at an elevated temperature thatis below the melting point of the polyethylene, preferably at about 135°C., wherein the polyethylene has reduced free radical content andpreferably has no residual free radical detectable by electron spinresonance.

[0033] In accordance with another aspect of the invention, there isprovided a method of deforming polyethylene, wherein the temperature isless than the melting point of the polyethylene and above roomtemperature, preferably between about 100° C. and about 137° C., morepreferably between about 120° C. and about 137° C., yet more preferablybetween about 130° C. and about 137° C., and most preferably at about135° C.

[0034] Yet in another aspect of the present invention, there is provideda method of treating irradiated crosslinked polyethylene composition inorder to reduce the residual free radicals comprising steps of:mechanically deforming the polyethylene; and annealing at a temperaturethat is below the melting point of the polyethylene in order to reducethe thermal stresses, wherein the mechanical deformation is performed(preferably at about 135° C.), optionally in presence of a sensitizingenvironment.

[0035] Still in another aspect of the invention, there is provided anirradiated crosslinked polyethylene composition made by the processcomprising steps of: irradiating at a temperature that is below themelting point of the polyethylene; mechanically deforming thepolyethylene below the melting point of the irradiated polyethylene inorder to reduce the concentration of residual free radicals; annealingat a temperature above the melting point; and cooling down to roomtemperature.

[0036] In another aspect, the invention provides a method of making anirradiated crosslinked polyethylene composition comprising steps of:mechanically deforming the polyethylene at a solid- or a molten-state;crystallizing/solidifying the polyethylene at the deformed state;irradiating the polyethylene below the melting point of thepolyethylene; and heating the irradiated polyethylene below the meltingpoint in order to reduce the concentration of residual free radicals andto recover the original shape or preserve shape memory.

[0037] Still in another aspect, the invention provides an irradiatedcrosslinked polyethylene composition made by the process comprisingsteps of: mechanically deforming the polyethylene at a solid- or amolten-state; crystallizing/solidifying the polyethylene at the deformedstate; irradiating the polyethylene below the melting point of thepolyethylene; and heating the irradiated polyethylene below the meltingpoint in order to reduce the concentration of residual free radicals andto recover the original shape or preserve shape memory.

[0038] Still in another aspect, the invention provides an irradiatedcrosslinked polyethylene with substantially reduced or no detectableresidual free radicals, wherein crystallinity of the polyethylene isabout 51% or greater.

[0039] These and other aspects of the present invention will becomeapparent to the skilled person in view of the description set forthbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1. FIG. 1 shows schematically the channel die set-up used inpreparing some of the samples described in the Examples disclosedherein. The test sample A is first heated to a desired temperature alongwith the channel die B. The channel die B is then placed in acompression molder and the heated sample A is placed and centered in thechannel. The plunger C, which is also preferably heated to the sametemperature, is placed in the channel. The sample A is then compressedby pressing the plunger C to the desired compression ratio. The flowdirection (FD), wall direction (WD), and compression direction (CD) areas marked.

[0041]FIG. 2. FIG. 2 shows schematically the oxidative aging oraccelerated aging process and determination of residual free radicalsthereafter. A specimen is prepared by cutting a 3 mm by 3 mm by 10 mmpiece near the body center with long axis of the specimen in the flowdirection of the channel die (see A). The specimen is then analyzed withelectron spin resonance for residual free radicals. The remaining halfof the test sample is further machined to obtain a cube with dimensionsof 1 cm by 1 cm by 1 cm. This cubic specimen (see B) is then subjectedto thermo-oxidative aging or accelerated aging in air convection oven at80° C. for three weeks. This method of aging will induce oxidation inthe polyethylene if there are residual free radicals. At the completionof the aging, the cubic specimen is cut in half and microtomed to removea 200 micrometer thin section. The section is then analyzed using aBioRad UMA500 infra-red microscope as a function of depth away from theedge of the microtomed section as shown with arrow in the figure.

DETAILED DESCRIPTION OF THE INVENTION

[0042] The present invention describes methods that allow reduction inthe concentration of residual free radicals in irradiated polyethylene,preferably to undetectable levels, without heating the material aboveits melting point. This method involves contacting the irradiatedpolyethylene with a sensitizing environment, and heating thepolyethylene to above a critical temperature that allows the freeradicals to react with the sensitizing environment, but is still belowthe melting point. It is likely that this critical temperaturecorresponds to the alpha transition of the polyethylene. The alphatransition of polyethylene is normally around 90-95° C.; however, in thepresence of a sensitizing environment that is soluble in polyethylene,the alpha transition may be depressed. The alpha transition is believedto induce motion in the crystalline phase, which is believed to increasethe diffusion of the sensitizing environment into this phase and/orrelease the trapped free radicals. The free radicals can now react withthe sensitizing gas and/or liquid, which are thought to act as a linkingagent between adjacent free radicals.

[0043] The material resulting from the present invention is acrosslinked polyethylene that has reduced residual free radicals, andpreferably no detectable free radicals, while not substantiallycompromising the crystallinity and modulus.

[0044] According to the invention, the polyethylene is irradiated inorder to crosslink the polymer chains. In general, gamma irradiationgives a high penetration depth but takes a longer time, resulting in thepossibility of some oxidation. In general, electron irradiation givesmore limited penetration depths but takes a shorter time, and hence thepossibility of oxidation is reduced. The irradiation dose can be variedto control the degree of crosslinking and crystallinity in the finalpolyethylene product. Preferably, a dose of greater than about 1 kGy isused, more preferably a dose of greater than about 20 kGy is used. Whenelectron irradiation is used, the energy of the electrons can be variedto change the depth of penetration of the electrons, thereby controllingthe degree of penetration of crosslinking in the final product.Preferably, the energy is about 0.5 MeV to about 10 MeV, more preferablyabout 5 MeV to about 10 MeV. Such variability is particularly usefulwhen the irradiated object is an article of varying thickness or depth,for example, an articular cup for a medical prosthesis.

[0045] The invention also provides an improved irradiated crosslinkedpolyethylene, containing reduced free radical concentration andpreferably containing substantially no detectable free radicals, made bythe process comprising steps of contacting the irradiated polyethylenewith a sensitizing environment; annealing at a temperature that is belowthe melting point of the polyethylene; and elevating to a temperaturethat is below the melting point in presence of a sensitizing environmentin order to reduce the concentration of residual free radicals,preferably to an undetectable level.

[0046] The present invention provides methods of treating polyethylene,wherein crystallinity of the polyethylene is higher than that of thestarting unirradiated polyethylene or irradiated polyethylene that hasbeen melted, wherein crystallinity of the polyethylene is at least about51%, wherein elastic modulus of the polyethylene is about the same as oris higher than that of the starting unirradiated polyethylene.

[0047] The present invention also describes methods that allow reductionin the concentration of residual free radical in irradiatedpolyethylene, even to undetectable levels, without heating the materialabove its melting point. This method involves subjecting an irradiatedsample to a mechanical deformation that is below the melting point. Thedeformation temperature could be as high as about 135° C. Thedeformation causes motion in the crystalline lattice, which permitsrecombination of free radicals previously trapped in the lattice throughcrosslinking with adjacent chains or formation of trans-vinyleneunsaturations along the back-bone of the same chain. If the deformationis of sufficiently small amplitude, plastic flow can be avoided. Thepercent crystallinity should not be compromised as a result.Additionally, it is possible to perform the mechanical deformation onmachined components without loss in mechanical tolerance. The materialresulting from the present invention is a crosslinked polyethylene thathas reduced concentration of residuals free radical, and preferablysubstantially no detectable free radicals, while not substantiallycompromising the crystallinity and modulus.

[0048] The present invention further describes that the deformation canbe of large magnitude, for example, a compression ratio of 2 in achannel die. The deformation can provide enough plastic deformation tomobilize the residual free radicals that are trapped in the crystallinephase. It also can induce orientation in the polymer that can provideanisotropic mechanical properties, which can be useful in implantfabrication. If not desired, the polymer orientation can be removed withan additional step of annealing at an increased temperature below orabove the melting point.

[0049] According to another aspect of the invention, a high straindeformation can be imposed on the irradiated component. In this fashion,free radicals trapped in the crystalline domains likely can react withfree radicals in adjacent crystalline planes as the planes pass by eachother during the deformation-induced flow. High frequency oscillation,such as ultrasonic frequencies, can be used to cause motion in thecrystalline lattice. This deformation can be performed at elevatedtemperatures that is below the melting point of the polyethylene, andwith or without the presence of a sensitizing gas. The energy introducedby the ultrasound yields crystalline plasticity without an increase inoverall temperature.

[0050] The present invention also provides methods of further annealingfollowing free radical elimination below melting point. According to theinvention, elimination of free radicals below the melt is achievedeither by the sensitizing gas methods and/or the mechanical deformationmethods. Further annealing of crosslinked polyethylene containingreduced or no detectable residual free radicals is done for variousreasons, for example:

[0051] 1. Mechanical deformation, if large in magnitude (for example, acompression ratio of two during channel die deformation), will inducemolecular orientation, which may not be desirable for certainapplications, for example, acetabular liners. Accordingly, formechanical deformation:

[0052] a) Annealing below the melting point (for example, less thanabout 137° C.) is utilized to reduce the amount of orientation and alsoto reduce some of the thermal stresses that can persist following themechanical deformation at an elevated temperature and cooling down.Following annealing, it is desirable to cool down the polyethylene atslow enough cooling rate (for example, at about 10° C./hour) so as tominimize thermal stresses. If under a given circumstance, annealingbelow the melting point is not sufficient to achieve reduction inorientation and/or removal of thermal stresses, one can heat thepolyethylene to above its melting point.

[0053] b) Annealing above the melting point (for example, more thanabout 137° C.) can be utilized to eliminate the crystalline matter andallow the polymeric chains to relax to a low energy, high entropy state.This relaxation will lead to the reduction of orientation in the polymerand will substantially reduce thermal stresses. Cooling down to roomtemperature is then carried out at a slow enough cooling rate (forexample, at about 10° C./hour) so as to minimize thermal stresses.

[0054] 2. The contact before, during, and/or after irradiation with asensitizing environment to yield a polyethylene with no substantialreduction in its crystallinity when compared to the reduction incrystallinity that otherwise occurs following irradiation and subsequentmelting. The crystallinity of polyethylene contacted with a sensitizingenvironment and the crystallinity of radiation treated polyethylene isreduced by annealing the polymer above the melting point (for example,more than about 137° C.). Cooling down to room temperature is thencarried out at a slow enough cooling rate (for example, at about 10°C./hour) so as to minimize thermal stresses.

[0055] As described herein, it is demonstrated that mechanicaldeformation can eliminate residual free radicals in a radiationcrosslinked UHMWPE. The invention also provides that one can firstdeform UHMWPE to a new shape either at solid- or at molten-state, forexample, by compression. According to a process of the invention,mechanical deformation of UHMWPE when conducted at a molten-state, thepolymer is crystallized under load to maintain the new deformed shape.Following the deformation step, the deformed UHMWPE sample is irradiatedbelow the melting point to crosslink, which generates residual freeradicals. To eliminate these free radicals, the irradiated polymerspecimen is heated to a temperature below the melting point of thedeformed and irradiated polyethylene (for example, up to about 135° C.)to allow for the shape memory to partially recover the original shape.Generally, it is expected to recover about 80-90% of the original shape.During this recovery, the crystals undergo motion, which can help thefree radical recombination and elimination. The above process is termedas a ‘reverse-IBMA’. The reverse-IBMA (reverse-irradiation below themelt and mechanical annealing) technology can be a suitable process interms of bringing the technology to large-scale production ofUHMWPE-based medical devices.

[0056] These and other aspects of the present invention will becomeapparent to the skilled person in view of the description set forthbelow.

[0057] A “sensitizing environment” refers to a mixture of gases and/orliquids (at room temperature) that contain sensitizing gaseous and/orliquid component(s) that can react with residual free radicals to assistin the recombination of the residual free radicals. The gases maybeacetylene, chloro-trifluoro ethylene (CTFE), ethylene, or like. Thegases or the mixtures of gases thereof may contain noble gases such asnitrogen, argon, neon and like. Other gases such as, carbon dioxide orcarbon monoxide may also be present in the mixture. In applicationswhere the surface of a treated material is machined away during thedevice manufacture, the gas blend could also contain oxidizing gasessuch as oxygen. The sensitizing environment can be dienes with differentnumber of carbons, or mixtures of liquids and/or gases thereof. Anexample of a sensitizing liquid component is octadiene or other dienes,which can be mixed with other sensitizing liquids and/or non-sensitizingliquids such as a hexane or a heptane. A sensitizing environment caninclude a sensitizing gas, such as acetylene, ethylene, or a similar gasor mixture of gases, or a sensitizing liquid, for example, a diene. Theenvironment is heated to a temperature ranging from room temperature toa temperature below the melting point of the material.

[0058] “Residual free radicals” refers to free radicals that aregenerated when a polymer is exposed to ionizing radiation such as gammaor e-beam irradiation. While some of the free radicals recombine witheach other to from crosslinks, some become trapped in crystallinedomains. The trapped free radicals are also known as residual freeradicals.

[0059] The phrase “substantially no detectable residual free radical”refers to no detectable free radical or no substantial residual freeradical, as measured by electron spin resonance (ESR). The lowest levelof free radicals detectable with state-of-the-art instruments is about10¹⁴ spins/gram and thus the term “detectable” refers to a detectionlimit of 10¹⁴ spins/gram by ESR.

[0060] The terms “about” or “approximately” in the context of numericalvalues and ranges refers to values or ranges that approximate or areclose to the recited values or ranges such that the invention canperform as intended, such as having a desired degree of crosslinkingand/or a desired lack of free radicals, as is apparent to the skilledperson from the teachings contained herein. This is due, at least inpart, to the varying properties of polymer compositions. Thus theseterms encompass values beyond those resulting from systematic error.

[0061] The terms “alpha transition” refers to a transitional temperatureand is normally around 90-95° C.; however, in the presence of asensitizing environment that dissolves in polyethylene, the alphatransition may be depressed. The alpha transition is believed (Anexplanation of the “alpha transition temperature” can be found inAnelastic and Dielectric Effects in Polymeric Solids, pages 141-143, byN. G. McCrum, B. E. Read and G. Williams; J. Wiley and Sons, N.Y., N.Y.,published 1967) to induce motion in the crystalline phase, which ishypothesized to increase the diffusion of the sensitizing environmentinto this phase and/or release the trapped free radicals.

[0062] The term “critical temperature” corresponds to the alphatransition of the polyethylene.

[0063] The term “below melting point” or “below the melt” refers to atemperature below the melting point of a polyethylene, for example,UHMWPE. The term “below melting point” or “below the melt” refers to atemperature less than 145° C., which may vary depending on the meltingtemperature of the polyethylene, for example, 145° C., 140° C. or 135°C., which again depends on the properties of the polyethylene beingtreated, for example, molecular weight averages and ranges, batchvariations, etc. The melting temperature is typically measured using adifferential scanning calorimeter (DSC) at a heating rate of 10° C. perminute. The peak melting temperature thus measured is referred to asmelting point and occurs, for example, at approximately 137° C. for somegrades of UHMWPE. It may be desirable to conduct a melting study on thestarting polyethylene material in order to determine the meltingtemperature and to decide upon an irradiation and annealing temperature.

[0064] The term “pressure” refers to an atmospheric pressure, above theambient pressure, of at least about 1 atm for annealing in a sensitizingenvironment.

[0065] The term “annealing” refers to heating the polymer below its peakmelting point. Annealing time can be at least 1 minute to several weekslong. In one aspect the annealing time is about 4 hours to about 48hours, preferably 24 to 48 hours and more preferably about 24 hours. Theannealing time required to achieve a desired level of recovery followingmechanical deformation is usually longer at lower annealingtemperatures. “Annealing temperature” refers to the thermal conditionfor annealing in accordance with the invention.

[0066] The term “contacted” includes physical proximity with or touchingsuch that the sensitizing agent can perform its intended function.Preferably, a polyethylene composition or pre-form is sufficientlycontacted such that it is soaked in the sensitizing agent, which ensuresthat the contact is sufficient. Soaking is defined as placing the samplein a specific environment for a sufficient period of time at anappropriate temperature. The environment include a sensitizing gas, suchas acetylene, ethylene, or a similar gas or mixture of gases, or asensitizing liquid, for example, a diene. The environment is heated to atemperature ranging from room temperature to a temperature below themelting point of the material. The contact period ranges from at leastabout 1 minute to several weeks and the duration depending on thetemperature of the environment. In one aspect the contact time period atroom temperature is about 24 hours to about 48 hours and preferablyabout 24 hours.

[0067] The term “Mechanical deformation” refers to a deformation takingplace below the melting point of the material, essentially‘cold-working’ the material. The deformation modes include uniaxial,channel flow, uniaxial compression, biaxial compression, oscillatorycompression, tension, uniaxial tension, biaxial tension, ultra-sonicoscillation, bending, plane stress compression (channel die) or acombination of any of the above. The deformation could be static ordynamic. The dynamic deformation can be a combination of the deformationmodes in small or large amplitude oscillatory fashion. Ultrasonicfrequencies can be used. All deformations can be performed in thepresence of sensitizing gases and/or at elevated temperatures.

[0068] The term “deformed state” refers to a state of the polyethylenematerial following a deformation process, such as a mechanicaldeformation, as described herein, at solid or at melt. Following thedeformation process, deformed polyethylene at a solid state or at meltis be allowed to solidify/crystallize while still maintains the deformedshape or the newly acquired deformed state.

[0069] “IBMA” refers to irradiation below the melt and mechanicalannealing. “IBMA” was formerly referred to as “CIMA” (Cold Irradiationand Mechanically Annealed).

[0070] Sonication or ultrasonic at a frequency range between 10 and 100kHz is used, with amplitudes on the order of 1-50 microns. The time ofsonication is dependent on the frequency and temperature of sonication.In one aspect, sonication or ultrasonic frequency ranged from about 1second to about one week, preferably about 1 hour to about 48 hours,more preferably about 5 hours to about 24 hours and yet more preferablyabout 12 hours.

[0071] By ultra-high molecular weight polyethylene (UHMWPE) is meantchains of ethylene that have molecular weights in excess of about500,000 g/mol, preferably above about 1,000,000 g/mol, and morepreferably above about 2,000,000 g/mol. Often the molecular weights canreach about 8,000,000 g/mol or more. By initial average molecular weightis meant the average molecular weight of the UHMWPE starting material,prior to any irradiation. See U.S. Pat. No. 5,879,400; PCT/US99/16070,filed on Jul. 16, 1999, WO 20015337, and PCT/US97/02220, filed Feb. 11,1997, WO 9729793, for properties of UHMWPE.

[0072] By “crystallinity” is meant the fraction of the polymer that iscrystalline. The crystallinity is calculated by knowing the weight ofthe sample (weight in grams), the heat absorbed by the sample in melting(E, in J/g) and the heat of melting of polyethylene crystals (ΔH=29IJ/g), and using the following equation:

% Crystallinity=E/w·ΔH

[0073] By tensile “elastic modulus” is meant the ratio of the nominalstress to corresponding strain for strains as determined using thestandard test ASTM 638 M III and the like or their successors.

[0074] The term “conventional UHMWPE” refers to commercially availablepolyethylene of molecular weights greater than about 500,000.Preferably, the UHMWPE starting material has an average molecular weightof greater than about 2 million.

[0075] By “initial average molecular weight” is meant the averagemolecular weight of the UHMWPE starting material, prior to anyirradiation.

[0076] The term “interface” in this invention is defined as the niche inmedical devices formed when an implant is in a configuration where thepolyethylene is in functional relation with another piece (such as ametallic or a polymeric component), which forms an interface between thepolymer and the metal or another polymeric material. For example,interfaces of polymer-polymer or polymer-metal in medical prosthesissuch as, orthopedic joints and bone replacement parts, e.g., hip, knee,elbow or ankle replacements. Medical implants containingfactory-assembled pieces that are in intimate contact with thepolyethylene form interfaces. In most cases, the interfaces are notaccessible to the ethylene oxide (EtO) gas or the gas plasma (GP) duringa gas sterilization process.

[0077] The piece forming an interface with polymeric material can bemetallic. The metal piece in functional relation with polyethylene,according to the present invention, can be made of a cobalt chromealloy, stainless steel, titanium, titanium alloy or nickel cobalt alloy,for example.

[0078] The products and processes of this invention also apply tovarious types of polymeric materials, for example,high-density-polyethylene, low-density-polyethylene,linear-low-density-polyethylene, UHMWPE, and polypropylene.

[0079] The invention is further demonstrated by the following example,which do not limit the invention in any manner.

EXAMPLES Example 1 Channel Die Set-Up in Sample Preparation

[0080] Referring to FIG. 1, a test sample ‘A’ is first heated to adesired temperature along with the channel die B. The channel die ‘B’ isthen placed in a compression molder and the heated sample A is placedand centered in the channel. The plunger ‘C’, which also is preferablyheated to the same temperature, is placed in the channel. The sample ‘A’is then compressed by pressing the plunger ‘C’ to the desiredcompression ratio. The sample will have an elastic recovery afterremoval of load on the plunger. The compression ratio, λ (finalheight/initial height), of the test sample is measured after the channeldie deformation following the elastic recovery. The flow direction (FD),wall direction (WD), and compression direction (CD) are as marked inFIG. 1.

Example 2 Warm Irradiation with Sensitizing Gas Below the AlphaTransition

[0081] Test samples or a finished medical product of ultra-highmolecular weight polyethylene (UHMWPE) are placed in a gas impermeablepouch (such as polyethylene laminated aluminum foil), purged with asensitizing gas and sealed with sensitizing gas substantially fillingthe package. The package is then heated to a temperature between roomtemperature and 90° C. The package is then irradiated at the heatedtemperature using e-beam or gamma irradiation.

Example 3 Warm Irradiation with Sensitizing Gas Below the AlphaTransition with Subsequent Annealing in Sensitizing Gas

[0082] Test samples or a finished medical product of UHMWPE are placedin a gas impermeable pouch (such as polyethylene laminated aluminumfoil), purged with a sensitizing gas and sealed with sensitizing gassubstantially filling the package. The package is then heated to atemperature between room temperature and 90° C. The package is thenirradiated at the heated temperature using e-beam or gamma irradiation.The package is then annealed at a temperature that is below the meltingpoint of polyethylene.

Example 4 Warm Irradiation with Sensitizing Gas Above the AlphaTransition and Below the Melting Point

[0083] Test samples of UHMWPE are placed in a gas impermeable pouch(such as polyethylene laminated aluminum foil), purged with asensitizing gas and sealed with sensitizing gas substantially fillingthe package. The package is then heated to a temperature between 90° C.and melting temperature (about 145° C.). The package is then irradiatedat the heated temperature using e-beam or gamma irradiation.

Example 5 Warm Irradiation with Sensitizing Gas Above the AlphaTransition and Below the Melting Point with Subsequent Annealing inSensitizing Gas

[0084] Test samples of UHMWPE are placed in a gas impermeable pouch(such as polyethylene laminated aluminum foil), purged with asensitizing gas and sealed with sensitizing gas substantially fillingthe package. The package is then heated to a temperature between 90° C.and melting temperature (about 145° C.). The package is then irradiatedat the heated temperature using e-beam or gamma irradiation. The packageis then annealed at a temperature that is below the melting of pointpolyethylene.

Example 6 Post-Irradiation Contacting with a 5%/95% Acetylene/NitrogenGas Blend at an Elevated Temperature to Reduce the Concentration ofResidual Free Radicals

[0085] GUR 1050 ram-extruded UHMWPE bar stock (3.5″ diameter) wasmachined into 4 cm thick cylinders. The cylinders were irradiated usingan Impela-10/50 AECL 10 MeV electron beam accelerator (E-Beam Services,Cranberry N.J.) to a dose level of 100 kGy in air. The irradiatedcylinders were machined into 2 mm thick sections. Test samples wereprepared using sections with dimensions of 3×3×2 mm. Test samples wereplaced in polyethylene laminated aluminum foil pouches (three testsamples per pouch). Three of the pouches were purged with a 5%acetylene/95% nitrogen gas mixture (BOC Gas, Medford, Mass.) by pullingvacuum, then back-filling the pouch with the gas blend three times. Thepouches were sealed and left in slightly positive pressure of theacetylene/nitrogen gas blend. A fourth pouch was purged using the samemethod with 100% nitrogen gas and sealed with a slightly positivepressure of nitrogen gas inside the package.

[0086] Two of the acetylene/nitrogen-filled pouches and thenitrogen-filled pouch were then placed in a convection oven at 100° C.for 24 hours. The other acetylene/nitrogen-filled pouch was kept atambient temperature for 24 hours. The pouches were then opened, and thetest samples were analyzed with electron spin resonance to determine theconcentration of residual free radicals in the specimens. A set of threeadditional test samples that were left in air at room temperature werealso analyzed using electron spin resonance. Results are shown in Table1.

[0087] The results show that the irradiated test samples left in the 5%acetylene/95% nitrogen gas blend at room temperature for 24 hours hadsubstantial residual free radicals, as did the test samples stored inair at room temperature for 24 hours. The test samples left in the 100%nitrogen gas at 100° C. for 24 hours showed a slight decrease inresidual free radical concentration. The test samples left in 5%acetylene/95% nitrogen gas blend at 100° C. for 24 hours had nosubstantially detectable residual free radical. Therefore, the additionof 5% acetylene into nitrogen is sufficient to reduce the concentrationof the residual free radicals to undetectable levels following 100 kGyof electron beam irradiation. TABLE 1 Concentration of residual freeradicals measured in various specimens (n = 3 for all). E-Beam Post-Post-Irradiation Free radical Dose Irradiation Temperature Annealingconcentration Test sample (kGy) Environment (° C.) time (hrs) [10¹⁵spins/gram] As-Is following irradiation 100 Air 25 Not applicable 8.67 ±2.1 100% Nitrogen 100 100% nitrogen 100 24 3.99 ± 1.1 environment, 100°C. for 24 hours 5%/95% acetylene/nitrogen 100 5% acetylene 25 24 9.70 ±0.2 gas environment, room temperature 5%/95% acetylene/nitrogen 100 5%acetylene 100 24 Not detectable gas environment, 100° C. for 24 hoursFIRST RUN 5%/95% acetylene/nitrogen 100 5% acetylene 100 24 Notdetectable gas environment, 100° C. for 24 hours REPEAT RUN

Example 7 Irradiation of a Finished Polyethylene Medical Device in thePresence of a Sensitizing Gas at Room Temperature

[0088] A medical device is prepared from conventional UHMWPE andpackaged in a gas permeable material (such as Tyvek). It is then placedin gas impermeable packaging (such as foil laminated packaging). Thispackage is then purged several times using a sensitizing atmosphere andwas sealed in that atmosphere. The entire assembly is then irradiatedusing gamma irradiation or e-beam to a dose level of 1 to 1000 kGy.Following irradiation, the entire assembly is annealed. The annealingtemperature is selected such that the packaging remains intact and thatat least one level of hermetic seal between the outside and thecomponent is not broken to maintain sterility of the medical devicecomponent. The component is then shipped for surgical use. If sodesired, the remaining sensitizing gas is removed before shipping. Theremoval of the sensitizing gas is carried out by puncturing the package;or by removing the outer foil pouch and shipping the component in thegas permeable inner package.

Example 8 Reduction of Residual Free Radicals in a Finished PolyethyleneMedical Device

[0089] A medical device made out of polyethylene with residual freeradicals is placed in a sensitizing atmosphere and annealed in theatmosphere that is below the melting point of the polyethylene in orderto reduce the concentration of residual free radicals to at leastsubstantially undetectable levels.

Example 9 Channel Die Deformation of Irradiated Polyethylene

[0090] Test samples of ultra-high molecular weight polyethylene areirradiated at room temperature using e-beam or gamma radiation. Thesamples are then placed in a channel die at 120° C., and are deformed inuniaxial compression deformation by a factor of 2. The residual freeradical concentration, as measured with electron spin resonance, arecompared with samples held at 120° C. for the same amount of time.

Example 10 Channel Die Deformation of Irradiated Polyethylene Contactedwith a Sensitizing Environment

[0091] Test samples of ultra-high molecular weight polyethylene areirradiated at room temperature using e-beam or gamma radiation. Thesamples are contacted with a sensitizing gas, such as acetylene untilsaturated. The samples are then placed in a channel die at 120° C., andare deformed in uniaxial compression deformation by a factor of 2. Theresidual free radical concentration, as measured with electron spinresonance, are compared with samples held at 120° C. for the same amountof time.

Example 11 Warm Irradiation with Mechanical Annealing

[0092] Test samples of ultra-high molecular weight polyethylene areirradiated at 120° C. adiabatically (that is, without significant heatloss to the environment) with electron beam radiation. The samples arethen placed in a channel die at 120° C., and are deformed in uniaxialcompression deformation by a factor of 2. The residual free radicalconcentration, as measured with electron spin resonance, is comparedwith samples held at 120° C. for the same amount of time.

Example 12 Post-Irradiation Annealing in the Presence of 5%/95%Acetylene/Nitrogen Gas Mixed at an Elevated Temperature to Reduce theConcentration of Residual Free Radicals in a Large Polyethylene TestSample

[0093] GUR 1050 ram-extruded UHMWPE bar stock (3.5″ diameter) wasmachined into 4 cm thick cylinders. The cylinders were irradiated usingan Impela-10/50 AECL 10 MeV electron beam accelerator (E-Beam Services,Cranberry N.J.) to a dose level of 75 kGy in air. The irradiatedcylinders were machined into test samples with dimensions of about 2×2×2cm cubes. Two test samples were placed in two separate polyethylenelaminated aluminum foil pouches. One pouch was purged with a 5%acetylene/95% nitrogen gas mixture (BOC Gas, Medford, Mass.) by pullingvacuum, then back-filling the pouch with the gas blend. The pouch wassealed and left in slightly positive pressure of the acetylene/nitrogengas blend. The second pouch was purged using the same method with 100%nitrogen gas and sealed with a slightly positive pressure of nitrogengas inside the package.

[0094] Both pouches were then placed in a convection oven at 133° C. for24 hours. The pouches were then opened, and the test samples werefurther machined to prepare specimens for analysis with electron spinresonance. These specimens were prepared near the body center of thetest samples.

[0095] The ESR analysis showed substantially no detectable free radicalsin the specimen prepared from the irradiated polyethylene that wasannealed while in contact with 5%/95% acetylene/nitrogen gas mixture.The specimen prepared from the test sample that was annealed in 100%nitrogen showed a free radical signal, which was quantified to represent6×10¹⁴ spins/gram.

[0096] This example shows that the presence of even low concentrationsof a sensitizing gas such as 5% acetylene can reduce the concentrationof residual free radicals in a large test sample with dimensions typicalof a polyethylene orthopedic implant without heating the said testsample to above its melting point. This reduction in free radicalconcentration is more than what is obtained by subjecting the sameirradiated polyethylene to an identical thermal history in the presenceof 100% nitrogen.

Example 13 Post-Irradiation Mechanical Deformation at an ElevatedTemperature to Reduce the Concentration of Residual Free Radicals

[0097] GUR 1050 compression molded UHMWPE bar stock was machined intocubes of 4×4×4 cm dimensions. The cubes were irradiated using an gammairradiation to a dose level of 75 kGy in nitrogen. The irradiated cubeswere machined into test samples with dimensions of 2×2×1 cm. Two testsamples were placed in an air convection oven and heated to 135° C. inair, overnight (about 10 hours or more). One of the test samples wasthen placed in aluminum channel die, which was heated to 135° C., andpressed to a compression ratio, λ, of about two. The pressure was thenreleased and the sample was left to cool down to room temperature. Theother test sample was simply removed from the convection oven andallowed to cool down to room temperature with no mechanical deformation.

[0098] Both of these test samples were further machined. The test samplethat was subjected to heating only was cut to remove a 5 mm long sliver(about 2×2 mm cross-section) from the body center. The other sample thatwas subjected to heating and channel die compression was cut to remove a5 mm long sliver (about 2×2 mm cross-section) from the body center. Thelong-axis of the sliver was parallel to the channel die flow direction.Both of these slivers were then analyzed with electron spin resonance.

[0099] The ESR analysis showed a free radical signal (which wasquantified to represent 2×10¹⁵ spins/gram) in the sliver that wasprepared from the test sample that was heated to 135° C. overnight. Incontrast, the sliver prepared from the test sample that was heated to135° C. overnight and mechanically deformed in the channel die (λ=2) atthat temperature showed no detectable residual free radicals. Thisexample confirms that mechanical deformation at an elevated temperaturereduces the concentration of residual free radicals.

Example 14 Determination of Crystallinity with Differential ScanningCalorimetry (DSC) Method

[0100] Differential scanning calorimetry (DSC) technique was used tomeasure the crystallinity of the polyethylene test samples. The DSCspecimens were prepared from the body center of the polyethylene testsample unless it is stated otherwise.

[0101] The DSC specimen was weighed with an AND GR202 balance to aresolution of 0.01 milligrams and placed in an aluminum sample pan. Thepan was crimped with an aluminum cover and placed in the TA instrumentsQ-1000 Differential Scanning Calorimeter. The specimen was first cooleddown to 0° C. and held at 0° C. for five minutes to reach thermalequilibrium. The specimen was then heated to 200° C. at a heating rateof 10° C./min.

[0102] The enthalpy of melting measured in terms of Joules/gram was thencalculated by integrating the DSC trace from 20° C. to 160° C. Thecrystallinity was determined by normalizing the enthalpy of melting bythe theoretical enthalpy of melting of 100% crystalline polyethylene(291 Joules/gram). As apparent to the skilled person, other appropriateintegration also can be employed in accordance with the teachings of thepresent invention.

[0103] The average crystallinity of three specimens obtained from nearthe body center of the polyethylene test sample is recorded with astandard deviation.

[0104] The Q1000 TA Instruments DSC is calibrated daily with indiumstandard for temperature and enthalpy measurements.

Example 15 Crystallinity Measurements of Polyethylene FollowingIrradiation and Channel Die Deformation at an Elevated Temperature

[0105] GUR 1050 compression molded UHMWPE bar stock was machined intocubes of 4×4×4 cm dimensions. The cubes were irradiated using gammairradiation to a dose level of 75 kGy in nitrogen. The irradiated cubeswere machined into test samples with dimensions of 2×2×1 cm. One testsample (CIMA-12) was placed in an air convection oven and heated to 135°C. in air, overnight (10 hours). The test sample was then placed in analuminum channel die, which was heated to 135° C., and pressed to acompression ratio, λ, of about two. The pressure was then released andthe sample was left to cool down to room temperature.

[0106] The compressed test sample was further machined to preparespecimens from near the body center to be used to determine thecrystallinity. Three such specimens obtained from near the body centerwere analyzed using a TA instruments Differential Scanning Calorimeterat a heating rate of 10° C./min and a temperature scan range of 0° C. to200° C.

[0107] The enthalpy of melting (in terms of Joules/gram) was thencalculated by integrating the DSC trace from 20° C. to 160° C. Thecrystallinity was determined by normalizing the enthalpy of melting bythe theoretical enthalpy of melting of 100% crystalline polyethylene(291 Joules/gram).

[0108] The average crystallinity of the three specimens obtained fromnear the body center of the test sample was 58.9% with a standarddeviation of 0.7.

Example 16 Free Radical Concentration and Thermo-Oxidative Aging orAccelerated Aging Behavior of an Irradiated and Mechanically DeformedPolyethylene Sample

[0109] GUR 1050 compression molded UHMWPE bar stock was machined intocubes of 4×4×4 cm dimensions. The cubes were irradiated using gammairradiation to a dose level of 75 kGy in nitrogen. The irradiated cubeswere machined into test samples with dimensions of 2×2×1 cm. One testsample (CIMA-28) was placed in an air convection oven and heated to 135°C. in air for 4 hours. The test sample was then placed in an aluminumchannel die, which was heated to 135° C., and pressed to a compressionratio, λ, of about two. The pressure was then released and the samplewas put back into the air convection oven and heated for an additional 4hours at 135° C. to recover most of the plastic deformation.

[0110] A specimen was prepared by cutting a 3×3×10 mm piece near thebody center with long axis of the specimen in the flow direction of thechannel die (see A in FIG. 2). The specimen was analyzed with electronspin resonance and no free radicals were detected. The remaining half ofthe test sample was further machined to obtain a cube with dimensions of1××1 cm. This cubic specimen (see B in FIG. 2) was then subjected tothermo-oxidative aging or accelerated aging in air convection oven at80° C. for three weeks. This method of aging will induce oxidation inthe polyethylene if there are residual free radicals. At the completionof the aging, the cubic specimen was cut in half and microtomed toremove a 200 micrometer thin section. The section was then analyzedusing a BioRad UMA500 infra-red microscope as a function of depth awayfrom the edge of the microtomed section as shown in FIG. 2. Theinfra-red spectra collected with this method showed no detectablecarbonyl vibration throughout the microtomed section, indicating nodetectable oxidation. The crystallinity of the aged test sample was alsodetermined using three specimens cut form the said aged test sampleusing the DSC method described above in Example 14. The crystallinity ofthe three specimens averaged 59.2% with a standard deviation of 0.9 whenthe melting enthalpy was calculated by integrating the DSC trace from20° C. to 160° C.

[0111] The aging method provided additional support for the electronspin resonance in showing that irradiation followed by mechanicaldeformation at an elevated temperature results in a marked reduction inthe concentration of residual free radicals and an increase inthermo-oxidative stability of irradiated polyethylene.

Example 17 Annealing Following Free Radical Reduction Using Channel DieCompression at an Elevated Temperature

[0112] GUR 1050 UHMWPE bar stock was irradiated with gamma rays to 75kGy in nitrogen. The irradiated block was then machined to blocks withdimensions of 2×2×1 cm. Two of these blocks were placed in an airconvection oven at 133° C. for 4 hours. Both of these heated blocks werethen compressed in a channel die that was heated to 133° C. Thecompression ratio, λ=initial height/final height, was about two. Thedimensions of these blocks were measured and recorded after they werecooled down to room temperature (see Table 2).

[0113] One of the blocks (Block I in Table 2) was then annealed under noload at 135° C. for 16 hours and cooled down to room temperature.Following this annealing cycle the dimensions of the block were measuredagain as shown in the Table 2. This observation shows that the plasticdeformation was markedly recovered by annealing below the melting point.

[0114] The other block (Block II in Table 2) was annealed under no loadat 150° C. for 6 hours and cooled down to room temperature. Followingthis annealing cycle the dimensions of the block were measured again asshown in Table 2. This observation shows that plastic deformation isalmost fully recovered by annealing above the melting point. TABLE 2Annealing below and above melt using channel die compression at anelevated temperature. *Dimensions CD/FD/WD (mm) Following channel dieFollowing Sample Initial (mm) compression Annealing Block I 20 × 20 ×9.5 12 × 35 × 10 16.5 × 23.5 × 9.5 (Annealed below the melt) Block II 20× 20 × 9.5 10 × 40 × 10 20 × 20 × 9.5 (Annealed above the melt)

Example 18 Thermal Oxidative or Accelerated Aging Behavior of IrradiatedCross-Linked Polyethylenes That are Heated and Mechanically DeformedVersus an Irradiated Cross-Linked Heated Polyethylene

[0115] GUR 1050 UHMWPE bar stock was machined into blocks that were9×9×4 cm. The blocks were gamma irradiated in a vacuum package to 100kGy. Blocks were subsequently machined into the 19 mm cubes.

[0116] Four groups of cubes (n=2 for each temperature) were heated forone hour at 125° C., 128° C., 132° C., or 135° C., respectively.Subsequently, each heated cube was mechanically deformed between twoflat aluminum plates held at room temperature to a compression ratio, λ,of 4.5. The compression displacement was held at this point for 5minutes to allow for stress relaxation to occur. The load required tohold the displacement constant at this point was monitored. By the endof the five minutes the load had decreased and reached a steady state,at which point the sample was removed from the press. All deformed cubeswere then annealed at 135° C. for 1 hour to partially recoverdeformation. Samples were then machined in half in the direction ofcompression to expose an internal surface for accelerated aging.

[0117] Another four groups of cubes (n=2 for each group) were preparedto serve as thermal controls with no deformation history. These cubeswere subjected to the same thermal histories as those of the four groupsdescribed above. That is, the four groups were heated for one hour at125° C., 128° C., 132° C., or 135° C., respectively. The cubes were thenallowed to cool down to room temperature and annealed at 135° C. for 1hour. The thermal control samples were then machined in half in thedirection of compression to expose an internal surface for acceleratedaging.

[0118] The accelerated aging test specimens were placed in an airconvection oven at 80° C. and aged for 6 weeks. At the completion ofaging, the samples were cut in half and a 200 μm thin section wasremoved. The thin section was scanned using a BioRad UMA 500 infraredmicroscope at 100 micrometer intervals as a function of distance awayfrom the exposed internal free surface that was in contact with airduring aging. The scans were used to find the location of the maximumcarbonyl vibration. The infrared spectrum collected at this maximumcarbonyl location was used to assign an oxidation index to that agedcube. The oxidation in index was calculated by normalizing the areaunder the carbonyl vibration to that under the 1370 cm⁻¹ vibration. Thehigher the oxidation in the sample, the stronger is the carbonylvibration and as a result higher is the oxidation index.

[0119] The oxidation indexes of the four groups of deformed samples wereless than 0.03. In contrast, the thermal control groups showed oxidationindexes of 1.3, 1.2, 1.2, and 1.3 for the pre-heat temperatures of 125°C., 128° C., 132° C., or 135° C., respectively.

[0120] Based on above results, it is concluded that heating alone (belowthe melting point) does not improve the oxidation resistance ofirradiated and cross-linked polyethylene to the same extent as heatingand subsequent deformation do.

[0121] It is to be understood that the description, specific examplesand data, while indicating exemplary aspects, are given by way ofillustration and are not intended to limit the present invention.Various changes and modifications within the present invention willbecome apparent to the skilled artisan from the discussion, disclosureand data contained herein, and thus are considered part of theinvention.

1. An irradiated crosslinked polyethylene composition made by theprocess comprising steps of: a) irradiating at a temperature that isbelow the melting point of the polyethylene; and b) mechanicallydeforming the polyethylene below the melting point of the irradiatedpolyethylene in order to reduce the concentration of residual freeradicals.
 2. The polyethylene of claim 1, wherein the deformedpolyethylene is crystallized at the deformed state.
 3. The polyethyleneof claim 2, wherein the polyethylene is annealed below the melting pointfollowing crystallization.
 4. The polyethylene of claim 1, wherein thepolyethylene has substantially no trapped residual free radicaldetectable by electron spin resonance.
 5. The polyethylene of claim 1,wherein crystallinity of the polyethylene is about equal to or higherthan that of the starting unirradiated polyethylene.
 6. The polyethyleneof claim 1, wherein crystallinity of the polyethylene is about equal toor higher than that of the starting irradiated polyethylene that hasbeen melted.
 7. The polyethylene of claim 1, wherein crystallinity ofthe polyethylene is at least about 51%.
 8. The polyethylene of claim 1,wherein elastic modulus of the polyethylene is about the same as orhigher than that of the starting unirradiated polyethylene.
 9. Thepolyethylene of claim 1, wherein elastic modulus of the polyethylene isabout the same as or higher than that of the starting irradiatedpolyethylene that has been melted.
 10. The polyethylene of claim 1,wherein starting polyethylene material is in the form of a consolidatedstock.
 11. The polyethylene of claim 1, wherein starting polyethylenematerial is a finished product.
 12. The polyethylene of claim 11,wherein the finished product is a medical prosthesis.
 13. Thepolyethylene of claim 1, wherein the polyethylene is a polyolefin. 14.The polyolefin of claim 13 is selected from a group consisting of alow-density polyethylene, high-density polyethylene, linear low-densitypolyethylene, ultra-high molecular weight polyethylene (UHMWPE), ormixtures thereof.
 15. The polyethylene of claim 1, wherein thepolyethylene is in intimate contact with a metal piece.
 16. The metalpiece of claim 15 is a cobalt chrome alloy, stainless steel, titanium,titanium alloy or nickel cobalt alloy.
 17. The polyethylene of claim 1,wherein the polyethylene is in functional relation with anotherpolyethylene or a metal piece, thereby forming an interface.
 18. Thepolyethylene of claim 17, wherein the interface is not accessible toethylene oxide gas or gas plasma.
 19. The polyethylene of claim 1,wherein the mechanical deformation is uniaxial, channel flow, uniaxialcompression, biaxial compression, oscillatory compression, tension,uniaxial tension, biaxial tension, ultra-sonic oscillation, bending,plane stress compression (channel die) or a combination thereof.
 20. Thepolyethylene of claim 1, wherein the mechanical deformation is performedby ultra-sonic oscillation at an elevated temperature that is below themelting point of the irradiated polyethylene.
 21. The polyethylene ofclaim 1, wherein the mechanical deformation is performed by ultra-sonicoscillation at an elevated temperature that is below the melting pointof the polyethylene in presence of a sensitizing gas.
 22. Thepolyethylene of claim 1, wherein the deforming temperature is less thanabout 140° C.
 23. An irradiated crosslinked polyethylene compositionmade by the process comprising steps of: a) contacting the polyethylenecomposition with a sensitizing environment; b) irradiating by gamma orelectron beam radiation; and c) subjecting the composition to atemperature that is below the melting point of the polyethylenecomposition while in the presence of a sensitizing environment in orderto reduce the content of free radicals.
 24. The polyethylene of claim23, wherein the polyethylene is contacted with a sensitizing environmentprior to irradiation.
 25. The polyethylene of claim 23, wherein thesensitizing environment is acetylene, chloro-trifluoro ethylene (CTFE),trichlorofluoroethylene, ethylene gas, or mixtures containing noblegases thereof.
 26. The noble gas according to claim 25, is selected froma group consisting of nitrogen, argon, helium, neon, and any inert gasknown in the art.
 27. The environment according to claim 25, wherein thegas is a mixture of acetylene and nitrogen.
 28. The environmentaccording to claim 25, wherein the mixture comprising about 5% by volumeacetylene and about 95% by volume nitrogen.
 29. The polyethylene ofclaim 23, wherein the sensitizing environment is dienes with differentnumber of carbons, or mixtures of liquids thereof.
 30. The polyethyleneof claim 23, wherein crystallinity of the polyethylene is at least about51%.
 31. An irradiated crosslinked polyethylene composition havingreduced free radical content, and wherein crystallinity of thepolyethylene is at least about 51%.
 32. A method of making an irradiatedcrosslinked polyethylene composition comprising steps of: a) irradiatingat a temperature that is below the melting point of the polyethylene;and b) mechanically deforming the polyethylene below the melting pointof the irradiated polyethylene in order to reduce the concentration ofresidual free radicals.
 33. The method of claim 32, wherein the deformedpolyethylene is crystallized at the deformed state.
 34. The methodaccording to claim 32, wherein annealing temperature is below themelting point of the polyethylene.
 35. The method according to claim 34,wherein the annealing temperature is less than about 145° C.
 36. Themethod according to claim 32, wherein irradiation is carried out usinggamma radiation or electron beam radiation.
 37. The method according toclaim 32, wherein irradiation is carried out at an elevated temperaturethat is below the melting temperature.
 38. The method according to claim32, wherein radiation dose level is between about 1 and about 10,000kGy.
 39. The method according to claim 32, wherein mechanicaldeformation is performed in presence of a sensitizing environment. 40.The method according to claim 32, wherein mechanical deformation isperformed at an elevated temperature that is below the melting point ofthe polyethylene.
 41. The method according to claim 32, whereinmechanical deformation is performed in presence of a sensitizing gas atan elevated to a temperature that is below the melting point of thepolyethylene.
 42. The method according to claim 32, wherein themechanical deformation is uniaxial, channel flow, uniaxial compression,biaxial compression, oscillatory compression, tension, uniaxial tension,biaxial tension, ultra-sonic oscillation, bending, plane stresscompression (channel die) or a combination thereof.
 43. The methodaccording to claim 42, wherein the mechanical deformation is performedby ultra-sonic oscillation at an elevated temperature that is below themelting point of the polyethylene.
 44. The method according to claim 42,wherein the mechanical deformation is performed by ultra-sonicoscillation at an elevated to a temperature that is below the meltingpoint of the polyethylene in presence of a sensitizing gas.
 45. Themethod according to claim 32, wherein the mechanical deformation isperformed at a temperature that is less than about 135° C.
 46. A methodof making an irradiated crosslinked polyethylene comprising steps of: a)contacting the polyethylene with a sensitizing environment; b)irradiating by gamma or electron beam radiation; and c) subjecting thecomposition to a temperature that is below the melting point of thepolyethylene composition while in the presence of a sensitizingenvironment in order to reduce the content of free radicals.
 47. Themethod according to claim 46, wherein the polyethylene is contacted witha sensitizing environment prior to irradiation.
 48. The method accordingto claim 46, wherein irradiation is carried out in air or inertenvironment.
 49. The method according to claim 46, wherein the annealingin presence of sensitizing environment is carried out at above anambient atmospheric pressure.
 50. The method according to claim 46,wherein the annealing in the presence of sensitizing environment iscarried out at above an ambient atmospheric pressure of at last about1.0 atm.
 51. The method according to claim 46, wherein the annealing inthe presence of sensitizing environment is carried with high frequencysonication.
 52. The method according to claim 46, wherein thesensitizing environment is acetylene, trichlorofluoroethylene, ethylenegas, or mixtures of gases thereof.
 53. The method according to claim 46,wherein the sensitizing environment is dienes with different number ofcarbons, or mixtures of liquids thereof.
 54. The method according toclaim 46, wherein the sensitizing environment is a mixture of acetyleneand nitrogen.
 55. The method according to claim 46, wherein thesensitizing environment comprising about 5% by volume acetylene andabout 95% by volume nitrogen.
 56. An irradiated crosslinked polyethylenecomposition made by the process comprising steps of: a) irradiating at atemperature that is below the melting point of the polyethylene; b)mechanically deforming the polyethylene below the melting point of theirradiated polyethylene in order to reduce the concentration of residualfree radicals; c) annealing above the melting point; and d) cooling downto room temperature.
 57. A method of making an irradiated crosslinkedpolyethylene composition comprising steps of: a) mechanically deformingthe polyethylene at a solid- or a molten-state; b) crystallizing thepolyethylene at the deformed state; c) irradiating the polyethylene thatis below the melting point of the polyethylene; and d) heating theirradiated polyethylene below the melting point in order to reduce theconcentration of residual free radicals and to recover the originalshape.
 58. The method according to claim 57, wherein the irradiatedcrosslinked and heated polyethylene contains substantially reduced or nodetectable residual free radicals, wherein crystallinity of thepolyethylene is about 51% or greater.
 59. The method according to claim57, wherein the polyethylene has higher oxidation resistance thanirradiated and heated polyethylene.
 60. An irradiated crosslinkedpolyethylene composition made by the process comprising steps of: a)mechanically deforming the polyethylene at a solid- or a molten-state;b) crystallizing the polyethylene at the deformed state; c) irradiatingthe polyethylene that is below the melting point of the polyethylene;and d) heating the irradiated polyethylene below the melting point inorder to reduce the concentration of residual free radicals and torecover the original shape.
 61. The polyethylene of claim 60, whereinthe polyethylene contains substantially reduced or no detectableresidual free radicals, wherein crystallinity of the polyethylene isabout 51% or greater.
 62. The method according to claim 60, wherein thepolyethylene has higher oxidation resistance than irradiated and heatedpolyethylene.